1. Field of the Invention
The present invention relates to a fluorescent tube power supply and a backlight using the fluorescent tube power supply.
2. Description of the Related Art
Voltage and current sometimes change on a demand side due to inconsistency between a supply side that generates power, transmits power, and a demand side on which great change in load occurs. When such change occurs, power of a power supply cannot be sufficiently ensured, and electrical products may not operate properly.
For instance, in a backlight used in a liquid crystal television, a direct current (DC) power supply for converting an output of a domestic commercial power supply to a direct current and an inverter for converting an output of the DC power supply to an alternating current (AC) are used as a power supply circuit. As the power input to the inverter reduces, the brightness of a screen becomes less than the preset brightness. As the power increases, the brightness of the screen becomes more than the preset brightness.
A dispersion of the brightness among backlights occurs when the impedance of the load in an inverter power supply such as the inverter and the fluorescent tube varies even if the input voltage or current, that is, power does not change. In the inverter, variation of the impedance is relatively found in a transformer and a resonance capacitor.
In particular, a discharge tube such as a Cold Cathode Fluorescent Lamp (CCFL) used in the backlight has negative resistance characteristics. That is, the current decreases as the voltage rises, and the current increases as the voltage lowers.
The impedance of the fluorescent tube such as CCFL differs depending on the current-carrying time, and also varies depending on each fluorescent tube.
On the other hand, the luminance of the backlight depends on a tube current or a tube power of the fluorescent tube, and thus a stable current supply or power supply is required for the fluorescent tube.
Under such circumstances, Japanese Laid-Open Patent Publication No. H10-283044 proposes a constant current power supply device in which a current detection resistor interposed in an input/output line of a constant current source is arranged to detect a current, and in which a correction resistor having the same temperature condition as the current detection resistor is arranged to perform temperature compensation.
Japanese Laid-Open Patent Publication No. S61-144108 proposes a constant current circuit for controlling a large current so that it remain a constant value, where a current supplied to a load is detected with voltage across a resistor element, and the detected voltage is fed back to an input side to supply a constant current to the load.
Japanese Laid-Open Patent Publication No. S55-105712 proposes a clip stabilizing circuit comprising a resistor arranged on a line between an input end and an output end, detecting a voltage across the resistor, and feeding back the voltage to an input side, thereby enhancing the accuracy.